1. Field of the Invention
The present invention relates to electric or electronic circuits which are generated on a curved surface, especially a doubly-curved surface. The invention is especially useful for generating circuits requiring high precision, robust materials (high temperature or harsh environments), or multilayered designs where the circuits are stacked or embedded in additional dielectric or conductive layers. The invention is capable of generating conductive and dielectric patterns on otherwise undevelopable (doubly curved) three-dimensional surfaces.
2. Description of Related Art
In the aerospace and defense industries conformal electrical circuits are desirable for a variety of electrical means with regards to electromagnetic energy and radio frequency (RF) energy. RF energy is commonly used for communications, may be used for imaging in the form of millimeter waves, and also includes various radar tracking frequencies. Circuit patterns may be placed on the surface, or embedded within, the structure of aircraft or other vehicles. The circuits are suitable for radome structures of aircraft and missiles, so that antennas or target seeking electronics may be environmentally protected while providing suitable transparency over the frequency band required by the electronics enclosed within the radome. Certain circuit designs or patterns are capable of transmitting specific frequencies of RF energy, while reflecting extraneous frequencies, thereby acting as an RF filter. Other circuit designs may have a broad shielding effect, acting as an EMI shield, where alternative designs may require multiple layers of patterned circuits of varying materials for reflective, transmissive, or attenuating functions. These are also suitable for surfaces which require a reduced radar cross section (stealth).
In the applications described above, the circuit must be of high precision and continuous for the circuit to perform as designed. Such circuits can be applied to a large variety of base substrates including, but not limited to: polymers, polymer composites, glasses, glass-ceramics, monolithic ceramics, reinforced ceramics, and ceramic matrix composites (CMCs). Patterning techniques and materials should be amenable to the substrate as well as the operating conditions experienced by the substrate, such as high temperatures generated during prolonged high speed flight of a missile.
The prior art of splicing flat sheets of etched conductors onto curved surfaces is undesirable as the pattern is not continuous, gaps are present at the joints of the spliced sheets, and electrical conductivity across the surface is not maintained. With composite structures requiring multiple circuit layers, registration between layers may be difficult to maintain as the composite materials are generally opaque and layup is generally performed by hand. Additionally, missile radome structures may be exposed to extreme heat (>600° C.) in service, damaging most polymeric substrates used in the splicing methodology, requiring an alternative approach.
In the prior art where an individual element is transferred from an optical mask onto a three-dimensional surface using laser ablation or exposure of photoresist and etching, i.e. photolithography, the speed at which elements can be generated on the part is limited by the speed of the multi-axis robot used to orient the patterning tool. This type of system is capable of producing seam free patterns in a step and repeat fashion, but the individual elements are subject to distortion by projection errors when the image is formed on the three-dimensional surface. As the curvature varies, the size of the individual elements will change substantially because of focal differences within the small field of view. Additionally, this technique is only amenable to repetitive patterns, and not for circuits requiring continuous, variable, or tailored patterns over large areas. This technique is insufficient for patterning on the inside of long, small diameter, missile radome structures as the robotic positioning system and image projection system is unable to orient normal to the interior surface due to space constraints, with focal distances being relatively short for high resolution features.